MTA heavily uses the Windows API and compiler extensions of the Microsoft compiler (MSVC). The most important extension is inline assembly (allows you to mix assembly and C++ code).
There's a gcc/clang extension for inline assembly too, but it uses a different syntax, hence huge parts of MTA had to be manually rewritten. From a technical perspective, it is possible to do that since there are pendants for almost all extensions/WinAPI functions on Linux. In theory, it's even possible to patch GTA:SA to use OpenGL and convert the executable format from PE (Windows) to ELF (Linux).
Practically, doing it is by far out of all proportion to the time and effort needed to implement it in comparison to just using Wine/virtualization instead. So finally, it's virtually impossible.

You can find all files that are directly related to the client here: https://github.com/multitheftauto/mtasa-blue/tree/master/Client
You can't. MTA never modifies IDE/IPL files, but directly accesses GTA:SA functionalities. If you need such a feature, you will have to implement it yourself.
I'm not exactly sure what you mean, but as far as I can tell MTA doesn't download any IDE or IPL files. It just behaves like the singleplayer and loads the "default map" on startup. MTA's .map files are an entirely different concept.
https://github.com/multitheftauto/mtasa-blue/tree/master/Client/core
When implementing all the stuff, please make sure you don't violate the GPL license MTA is licensed under. That means: If you change anything and publish it, you will have to publish all your source code changes as well.

The compiler has never been meant to be 100% secure. It can't even be fully secure since the MTA server still needs to read the script files.
However, it doesn't look like the decompiler in the video supports "obfuscation level 3" which is much better than the other two.

Actually, it's not about code changes by MTA (there are a few changes, but those are easy to migrate). The real problem is that the bytecode changed which is why compiled scripts won't work any longer. Additionally, some features (like fenvs) were removed.
The only way to get support for Lua 5.3 is to add a field to the meta.xml to switch between Lua versions. The question is though: Is the confusion and community fragmentation caused by this worth the few new features?

Your serial is not banned globally, so there's unfortunately nothing the MTA team could do about it. Therefore, your only chance to get unbanned is to kindly ask the server team to unban you.
(Also, don't try changing your serial using tools that pretend to do it as an attempt results in a global ban immediately and automatically)

Unfortunately, the ban log says you've been ban-evading several of our services lately. Therefore, you will not be unbanned very soon.
Also, using a VPN to hide your identity is not the best way to show regret. Please ask again in a few months.
*Note to Jusonex from Dutchman101: this guy is out of luck permanently, ccw also knows more..